Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignitio...Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignition system on large-bore dual-fuel marine engines especially under lean-burn condition,due to its advanced ignition stability and engine efficiency.However,the ignition and combustion mechanism in such dual-fuel pre-chamber engine is still unclear and the effects of in-cylinder swirl flow and mixture stratification on combustion require further investigation specifically.This paper numerically studied the detailed ignition mechanism and combustion process in a marine engine equipped with a pre-chamber ignition system,and revealed the flame development process in main chamber.Moreover,the effects of mixture stratification and swirl ratio on the combustion rate and further engine thermal efficiency are investigated under decoupled condition.The results mainly show that the jet flame develops along the pre-chamber orifice centerline at the initial stage and premixed combustion play an important role,while after that,heat release zone only exist at flame surface,and premixed flame propagation controls the combustion process.In addition,with higher swirl ratio the combustion rate increases significantly due to the wider ignition area.Mixture stratification degree plays a role in accelerating the combustion,either too high or too low stratification degree reduce the combustion rate,while a moderate stratification increases the combustion rate.And appropriate stratification degree by verifying the gas injection parameters can reduce fuel consumption in 0.3%.展开更多
In this paper,high-speed imaging and spectrometry diagnostics were used to study the spray flame structures and emission spectra of polyoxymethylene dimethyl ethers(PODE)in an optical constant volume vessel.The effect...In this paper,high-speed imaging and spectrometry diagnostics were used to study the spray flame structures and emission spectra of polyoxymethylene dimethyl ethers(PODE)in an optical constant volume vessel.The effects of oxygen volume fraction(15%to 80%)on some typical combustion characteristics such as spatially integrated natural intensity,flame propagation speed,lift-off length and the distribution of flame emission spectra were investigated.The results show that the PODE spray flame mainly exhibits blue chemiluminescence,which is different from the diesel yellow diffusion flame dominated by soot radiation.As the oxygen concentration increases,the flame natural luminosity and propagation speed increases,the position of lift-off region and flame tip move towards the injector nozzle,and the flame width becomes narrower.According to the spectral results,the blue chemiluminescence generated by carbon monoxide oxidation dominates the PODE flame luminescence,suggesting that no soot is formed.The increase in oxygen concentration leads to enlarged intensity and gradient of PODE flame radiation.In summary,the combination of PODE and oxygen-enriched combustion shows no soot formation and promotes the oxidation of carbon monoxide and unburned hydrocarbon.This study can provide more insights into PODE spray combustion,and offer guidelines for achieving efficient and clean combustion.展开更多
Effects of oxygen concentration on combustion and emissions of diesel engine are investigated by experiment.The intake oxygen concentration is controlled by adjusting CO2.The results show that very low levels of both ...Effects of oxygen concentration on combustion and emissions of diesel engine are investigated by experiment.The intake oxygen concentration is controlled by adjusting CO2.The results show that very low levels of both soot and NOx emissions can be achieved by modulating the injection pressure,tim-ing,and boost pressure at the low levels of oxygen concentration.However,both CO and HC emissions and fuel consumption distinctly increase at the low levels of oxygen concentration.The results also indicate that NOx emissions strongly depend on oxygen concentration,while soot emissions strongly depend on injection pressure.Decreasing oxygen concentration is the most effective method to control NOx emissions.High injection pressure is necessary to reduce smoke emissions.High injection pres-sure can also decrease the CO and HC emissions and improve engine efficiency.With the increase of intake pressure,both NOx and smoke emissions decrease.However,it is necessary to use the appro-priate intake pressure in order to get the low HC and CO emissions with high efficiency.展开更多
A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurement...A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurements are difficult to conduct directly because of the extremely high pressure and high temperature.In this study,back propagation(BP)neural network,BP optimized by mind evolution algorithm(MEA-BP)and BP neural network optimized by genetic algorithm(GA-BP)are established to determine the nonlinear temperature-dependent thermophysical properties of density,viscosity,and isobaric specific heat(C_(2))of hydrocarbon fuels at supercritical pressure.Meanwhile,approximate formulas for these properties prediction are primarily proposed using polynomial fitting.In this paper,models that can predict three types of physical properties of three kinds of hydrocarbon fuels and their mixtures in a wide temperature range under supercritical pressure are established.In the prediction of density and C_(2),BP neural network has a good prediction effect.The results show that the MAPE is lower than 2%in the prediction of density and C_(2),but the MAPE of viscosity prediction is slightly higher than 5%using BP.Furthermore,MEA and GA are used to optimize the prediction of viscosity.The optimization effect and computation of the MEA is better than that of GA because MEA does not have the local optimization and prematurity problems.The present work offers an efficient tool to predict the thermophysical properties of hydrocarbon fuels over a wide range of temperatures under supercritical pressure which can be easily extended to other fuels of interest.It will be beneficial to the experiment and numerical simulation studies of supercritical sprays.展开更多
This paper originally investigates the effect of NH_(3) dilution on soot formation when NH_(3) is gradually added into the fuel stream in an ethylene laminar diffusion flame stabilized on a Santoro burner.The variatio...This paper originally investigates the effect of NH_(3) dilution on soot formation when NH_(3) is gradually added into the fuel stream in an ethylene laminar diffusion flame stabilized on a Santoro burner.The variations of flame diameter and two flame heights,i.e.,mixture-strength flame height and visible flame height are carefully documented and analyzed.Moreover,local soot volume fraction(SVF)and soot temperature fields are simultaneously measured by compact-modulated absorption and emission technique,and the corresponding measurement random errors are also provided by the error propagation calculations for the first time.All the reported measurement random errors of SVF and soot temperature fields are estimated within the range of±0.07–±0.08 ppm and±40–±91 K,respectively.As an original database,the concomitantly measured SVF and soot temperature distributions are provided as high-fidelity datasets for refining soot formation model that is overrode by NH_(3).In addition,the flame cross-section average SVF F_(soot)(z)is calculated for every NH_(3) diluted flame,and the relative contributions of NH_(3) dilution and chemical effect are quantitatively assessed in terms of F_(max)-X_(NH_(3))plotting.It is found that when X_(NH_(3))<30%,the chemical effect of ammonia is about twice that of the dilution effect.While X_(NH_(3))>30%,the chemical effect and dilution effect of ammonia are gradually equal.Eventually,through modeling of the soot formation rate V in the flames,the relative contributions of chemical effect,dilution effect and thermal effect of NH_(3)are further novelty discriminated within the X_(NH_(3))from 0 to 46%and it is shown that NH_(3) chemical effect plays the dominate role in soot suppression,then the dilution effect and the thermal one at the least.展开更多
基金the financial supports provided by the Low-speed Marine Diesel Project(Project No.CDGC01-KT0308)National Natural Science Foundation of China(Grant No.91941102)。
文摘Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignition system on large-bore dual-fuel marine engines especially under lean-burn condition,due to its advanced ignition stability and engine efficiency.However,the ignition and combustion mechanism in such dual-fuel pre-chamber engine is still unclear and the effects of in-cylinder swirl flow and mixture stratification on combustion require further investigation specifically.This paper numerically studied the detailed ignition mechanism and combustion process in a marine engine equipped with a pre-chamber ignition system,and revealed the flame development process in main chamber.Moreover,the effects of mixture stratification and swirl ratio on the combustion rate and further engine thermal efficiency are investigated under decoupled condition.The results mainly show that the jet flame develops along the pre-chamber orifice centerline at the initial stage and premixed combustion play an important role,while after that,heat release zone only exist at flame surface,and premixed flame propagation controls the combustion process.In addition,with higher swirl ratio the combustion rate increases significantly due to the wider ignition area.Mixture stratification degree plays a role in accelerating the combustion,either too high or too low stratification degree reduce the combustion rate,while a moderate stratification increases the combustion rate.And appropriate stratification degree by verifying the gas injection parameters can reduce fuel consumption in 0.3%.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.51976134 and 91941102).
文摘In this paper,high-speed imaging and spectrometry diagnostics were used to study the spray flame structures and emission spectra of polyoxymethylene dimethyl ethers(PODE)in an optical constant volume vessel.The effects of oxygen volume fraction(15%to 80%)on some typical combustion characteristics such as spatially integrated natural intensity,flame propagation speed,lift-off length and the distribution of flame emission spectra were investigated.The results show that the PODE spray flame mainly exhibits blue chemiluminescence,which is different from the diesel yellow diffusion flame dominated by soot radiation.As the oxygen concentration increases,the flame natural luminosity and propagation speed increases,the position of lift-off region and flame tip move towards the injector nozzle,and the flame width becomes narrower.According to the spectral results,the blue chemiluminescence generated by carbon monoxide oxidation dominates the PODE flame luminescence,suggesting that no soot is formed.The increase in oxygen concentration leads to enlarged intensity and gradient of PODE flame radiation.In summary,the combination of PODE and oxygen-enriched combustion shows no soot formation and promotes the oxidation of carbon monoxide and unburned hydrocarbon.This study can provide more insights into PODE spray combustion,and offer guidelines for achieving efficient and clean combustion.
基金Supported by the National Natural Science Foundation of China(Grant Nos.50636040,50676066)
文摘Effects of oxygen concentration on combustion and emissions of diesel engine are investigated by experiment.The intake oxygen concentration is controlled by adjusting CO2.The results show that very low levels of both soot and NOx emissions can be achieved by modulating the injection pressure,tim-ing,and boost pressure at the low levels of oxygen concentration.However,both CO and HC emissions and fuel consumption distinctly increase at the low levels of oxygen concentration.The results also indicate that NOx emissions strongly depend on oxygen concentration,while soot emissions strongly depend on injection pressure.Decreasing oxygen concentration is the most effective method to control NOx emissions.High injection pressure is necessary to reduce smoke emissions.High injection pres-sure can also decrease the CO and HC emissions and improve engine efficiency.With the increase of intake pressure,both NOx and smoke emissions decrease.However,it is necessary to use the appro-priate intake pressure in order to get the low HC and CO emissions with high efficiency.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51922076 and 91941102)
文摘A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurements are difficult to conduct directly because of the extremely high pressure and high temperature.In this study,back propagation(BP)neural network,BP optimized by mind evolution algorithm(MEA-BP)and BP neural network optimized by genetic algorithm(GA-BP)are established to determine the nonlinear temperature-dependent thermophysical properties of density,viscosity,and isobaric specific heat(C_(2))of hydrocarbon fuels at supercritical pressure.Meanwhile,approximate formulas for these properties prediction are primarily proposed using polynomial fitting.In this paper,models that can predict three types of physical properties of three kinds of hydrocarbon fuels and their mixtures in a wide temperature range under supercritical pressure are established.In the prediction of density and C_(2),BP neural network has a good prediction effect.The results show that the MAPE is lower than 2%in the prediction of density and C_(2),but the MAPE of viscosity prediction is slightly higher than 5%using BP.Furthermore,MEA and GA are used to optimize the prediction of viscosity.The optimization effect and computation of the MEA is better than that of GA because MEA does not have the local optimization and prematurity problems.The present work offers an efficient tool to predict the thermophysical properties of hydrocarbon fuels over a wide range of temperatures under supercritical pressure which can be easily extended to other fuels of interest.It will be beneficial to the experiment and numerical simulation studies of supercritical sprays.
基金supported by the National Natural Science Foundation of China (Grant No.52130605)。
文摘This paper originally investigates the effect of NH_(3) dilution on soot formation when NH_(3) is gradually added into the fuel stream in an ethylene laminar diffusion flame stabilized on a Santoro burner.The variations of flame diameter and two flame heights,i.e.,mixture-strength flame height and visible flame height are carefully documented and analyzed.Moreover,local soot volume fraction(SVF)and soot temperature fields are simultaneously measured by compact-modulated absorption and emission technique,and the corresponding measurement random errors are also provided by the error propagation calculations for the first time.All the reported measurement random errors of SVF and soot temperature fields are estimated within the range of±0.07–±0.08 ppm and±40–±91 K,respectively.As an original database,the concomitantly measured SVF and soot temperature distributions are provided as high-fidelity datasets for refining soot formation model that is overrode by NH_(3).In addition,the flame cross-section average SVF F_(soot)(z)is calculated for every NH_(3) diluted flame,and the relative contributions of NH_(3) dilution and chemical effect are quantitatively assessed in terms of F_(max)-X_(NH_(3))plotting.It is found that when X_(NH_(3))<30%,the chemical effect of ammonia is about twice that of the dilution effect.While X_(NH_(3))>30%,the chemical effect and dilution effect of ammonia are gradually equal.Eventually,through modeling of the soot formation rate V in the flames,the relative contributions of chemical effect,dilution effect and thermal effect of NH_(3)are further novelty discriminated within the X_(NH_(3))from 0 to 46%and it is shown that NH_(3) chemical effect plays the dominate role in soot suppression,then the dilution effect and the thermal one at the least.
